1. Field of the Invention
The present invention relates to water-redispersible powders of film-forming polymers prepared from monomers containing ethylenic unsaturation.
2. Description of Related Art
Redispersible powders which are obtained by spray-drying of dispersions of acrylic film-forming polymers, especially of dispersions of vinyl ester polymers, are already known.
Film-forming polymers prepared from monomers containing ethylenic unsaturation are often employed as adjuvants in hydraulic inorganic binder compositions in order to improve their application and the properties after hardening, such as adhesion to various substrates, leakproofing, flexibility and mechanical properties.
When compared with aqueous dispersions, redispersbile powders offer the advantage of being capable of being premixed with cement in the form of ready-to-use pulverulent compositions that can be used, for example, for the manufacture of mortars and concretes intended to be attached to construction materials, for the manufacture of adhesive mortars or for the production of protective and decorative coatings inside or outside buildings.
It is customary to add relatively large quantities of inert substances and of protective colloids to powders, in order to succeed in obtaining powders that do not agglomerate during storage under the effect of pressure and temperature and that are conveniently redispersible in water.
To obtain readily redispersible polymer powders, it has been proposed to add to the dispersions, before the spraying, melamine-formaldehyde-sulphonate (U.S. Pat. No. 3,784,648) or naphthalene-formaldehyde-sulphonate (DE-A-3,143,070) condensation products and/or vinylpyrrolidone-vinyl acetate copolymers (EP-78,449).
French patent FR-A 2,245,723 is directed towards a stable and water-dispersible, freeze-dried preparation containing a powder of a polymer latex and a water-soluble dispersing agent which is a saccharide.